1. Field of the Invention
The present invention relates to a liquid crystal device and a method of driving the same. More particularly, it relates to such a liquid crystal optical device and a method of driving the same wherein formation of undesirable bends are unlikely in liquid crystal layered structure.
2. Description of the Prior Art
As compared with twisted liquid crystal displays broadly used, ferroelectric liquid crystal displays have attractive advantages such as quick response and wide viewing angles. A ferroelectric liquid crystal material is disposed between substrates in the form of a layered structure consisting of a number of liquid crystal layers. The layers of the liquid crystal are arranged in parallel to each other and normal to the substrate, and have a tendency of being bent between the substrates as illustrated in FIG. 1. The bends appear as undesirable optical defects in controlled molecular orientation at the positions where the directions of bend are changed, resulting in reduction of contrast of images. It is very difficult to remove such bends from the layered structure and the bends continue to degrade the contrast during its operation.
In order to reform the layered structure, the pretilt angle between the inside contiguous surface of the substrate and the directors (long axes) of the liquid crystal molecules has to be decreased as small as possible so that the molecules 13 become in parallel to the inside surface. Liquid crystal molecules have dielectric anisotropy .DELTA..epsilon. (.DELTA..epsilon.=.epsilon..sub.1 -.epsilon..sub.2 ; .epsilon..sub.1 is a dielectric constant in the direction parallel to molecular long axis; and .epsilon..sub.2 is a dielectric constant in the direction perpendicular to the molecular long axis). If the dielectric anisotropy .DELTA..epsilon. is negative, an electrical torque proportional to .DELTA..epsilon.E.sup.2 (E: external electric field) can be exerted upon the molecules in order to force the directors to be parallel to the surface by applying alternating electric fields between the substrates. The torque is exerted upon the molecules, however, in order to inversely erect the directors normal to the substrate if the dielectric anisotropy is positive. Accordingly, there have remained such bends in conventional structures resulting in poor contrast.
The viscosity of a liquid crystal material makes it furthermore difficult to reform the layered structure.
The volume of the liquid crystal material contracts between the substrate as the temperature is descended, often resulting in the formation of void spaces in the layered structure, called tree-like defects. If the viscosity of the liquid crystal is high, the formation of the void space is more likely. While a ferroelectric liquid crystal has a memory characteristic, the viscosity makes it difficult to change the optical property of the liquid crystal material at a selected pixel after the optical property thereof is continuously maintained for a relatively long time, resulting in an afterimage and a poor contrast.
Furthermore, orientation control films provided on the contiguous surfaces of the substrates have relatively large resistances and therefore tend to collect ionized impurities thereon or therein which have been charged with the opposite polarity to that of the spontaneous polarization of the ferroelectric liquid crystal. For this reason, there is formed an electric field in the liquid crystal layers even when no external voltage is applied between the substrates, resulting in a monostability and twisted orientation of the liquid crystal which inherently has a bistability.
The voltage level of the control signals applied to the liquid crystal display has been adjusted in accordance with the surrounding temperature to maintain driving performance. However, when the voltage level of the control signals is decreased at high driving temperatures, the electric force .DELTA..epsilon.E.sup.2 is decreased so that undesirable bends tend to remain.